Human squamous cell carcinomas (SCCs) arising from the skin and oral cavity can present in an indolent form, similar to hibernation, with no obvious tumor expansion or local/systemic invasion. SCC indolence, which restricts net tumor expansion and invasion at the primary site, is severely understudied. Very little is known about why indolent SCCs persist and when they evolve into an aggressive form. Without molecular and cellular markers of SCC indolence, they are either over-treated thus causing morbidity or under-treated thus resulting in increased death. To overcome the lack of matched indolent and evolved SCCs in humans for comparative studies, we created a mouse model system, in which tumors developed from the same primary tumor can enter into and break out of indolence. In this system, we targeted a KrasG12D mutant and Smad4 deletion, the two most frequent genetic alterations in human SCCs, into keratin K15+ stem cells. These mice rapidly develop SCCs. After transplanting these SCCs to the skin of immune compromised or immune competent mice, some grew similarly to parental tumors (non-indolent) while others became indolent. When indolent SCCs were out-of-indolence, they grew and metastasized rapidly. This unique model system provides matched indolent and out-of-indolence SCCs for longitudinal comparisons. Based on our preliminary data, we hypothesize: Breaking SCC indolence is not simply leaving proliferative dormancy or overcoming microenvironmental stress. Intrinsic tumor epithelial properties and microenvironmental influences that control self-renewal and motility of tumor cells are key contributors of breaking SCC indolence. Based on our preliminary data, Aim 1 will assess if the numbers and properties of a subset of tumor initiating cells (TICs) influence indolence. Aim 2 will assess if EMT-mediated TIC expansion and cell migration are sufficient to contribute to breaking indolence. Aim 3 will determine the net effect of tumor-associated macrophages and B lymphocytes on tumor indolence. In all three aims, cellular processes and molecules responsible for breaking SCC indolence in mice will be examined in human SCCs from our tumor bank to determine if these cellular and molecular markers correlate with non- or out-of-indolence behavior in human SCCs, thus predicting poor prognosis. Our unique model system, high throughput approaches and cross- species comparisons will significantly accelerate identification of markers associated with breaking SCC indolence. The proposed functional validation of candidate markers will simultaneously test therapeutic strategies to halt SCC evasion.